Brushless motors demand a greater torque per unit-current (expressed with the motor constant “Kt”) in order to obtain a higher power with a small body. For this purpose, in a conventional motor, coils are generally arranged to occupy an area not less than the area a permanent magnet occupies so that a number of coils—represented by S—in each phase becomes greater. However, in a case of arranging number of flat air-core coils on a printed circuit board, the increasing of number “S” incurs a higher manufacturing cost as well as more steps of processing. As a result, expensive brushless motors of a poor productivity are effected in many cases.
It has been known that a brushless motor conventionally having a stator employing “S” pieces of coils in each phase functions properly with only S/N pieces of coils in each phase. (Both “N” and “S/N” are integers.) Accordingly, it is well understood that reduction of a number of coils decreases a cost of components. However, when a total number of 3×S/N pieces of coils—where each phase includes S/N pieces of coils—are arranged around a rotary shaft with approximate equal spaces in between, there is a need to build another manufacturing apparatus for mounting coils to a printed circuit board which makes up a stator together with the coils. This requires expensive cost of apparatus and blocks the mass-production of low cost brushless motors.
FIGS. 4(a) and 4(b) are a cross sectional view and a perspective view of a conventional brushless motor. In the cross section of FIG. 4(a), total of 6 coils, i.e. a number of phases is 3, and coil No. 1–3 in each phase ×2, are fixedly mounted to printed circuit board 24 so that the coils face disc-shaped rotor 8 including a permanent magnet magnetized in number of polarities. On another side of the coils 1–3, flat-disc-shaped yoke 28 made of soft magnet material and coaxial with the permanent magnet is arranged for converging the magnetic flux travelling from the permanent magnet. Yoke 28 occupies an area not less than the area the permanent magnet occupies, and integrates with printed circuit board 24 which forms the stator. Screws 13 fix yoke 28 and printed circuit board 24 together to chassis 12.
The perspective view of FIG. 4(b) illustrates a brushless motor viewed from disc-shaped rotor 8 perspectively through respective components. Coils 1–3 are arranged in the entire circumference of the rotor so that the arranged places of coils correspond to the location of the permanent magnet. Printed circuit board 24 occupies an area not less than the area the coils occupy, and covers the entire circumference.
FIG. 2 illustrates a machine for bonding coils to a printed circuit board, this machine is one of appratuses for manufacturing the brushless motor discussed above. Numbers of jig bases 15 are mounted to the rim of rotary table 14. The cross section of FIG. 5(b) illustrates a status where coils 1–3 and printed circuit board 24 are mounted to jig bases 15 on the machine. The perspective view of FIG. 5(a) illustrates the same status shown in FIG. 5(b) viewed from the side of printed circuit board 24 perspectively through respective components.
In FIG. 2, some dozens of jig bases 15 are arranged on the rim of rotary table 14, and outside of the rim, stator-take-out-machine 16, coil-mounter 17, gluing machine 18 and printed-circuit-board-mounter 19 are arranged closely with each other in this order in the rotating direction of rotary table 14. From printed-circuit-board-mounter 19 to stator-take-out-machine 16, enough space is reserved. The rotation of rotary table 14 allows jig base 15 sequentially to call at stator-take-out-machine 16, coil mounter 17, gluing machine 18 and printed-circuit-board-mounter 19, and respective machines do their assigned jobs when they are called by jig base 15.
First, coil-mounter 17 mounts two sets of coils 1–3, namely total 6 coils, onto jig base 15. Second, gluing machine 18 applies enough glue to upper faces of each coil so that every coil is covered by the glue. Third, printed-circuit-board-mounter 19 mounts printed-circuit-board 24 onto the coils covered by the glue, then printed-circuit-board 24 and coils 1–3—both are set on jib-base 15 as well as pressed—are forwarded to stator-take-out-machine 16 whereby both are bonded by enough strength. Just before the stator-take-out-machine 16, the pressure applied to the coils and printed-circuit-board on jib-base 15 is released, then machine 16 takes out a stator formed of coils and printed circuit board. Vacant jig-base 15 is then forwarded to coil-mounter 17, and another cycle of process starts. Through the cycle of process discussed above, one jig-base 15 produces one stator, and jig-base 15 repeatedly rotates so that the stators are manufactured in volume.
In the brushless motor shown in FIG. 4, disc-shaped rotor 8 having a permanent magnet rotates with regard to yoke 28 fixed to chassis 12, thereby producing eddy-current-loss. Further, disc-shaped rotor 8 with a permanent magnet is attracted to yoke 28 fixed to chassis 12 by magnetic attraction whereby thrust receptor member 23 is urged to thrust bearing member 22. Meanwhile thrust bearing member 22 is provided to bearing 21 which supports shaft 20 disposed in disc-shaped rotor 8 mounted to chassis 12, and thrust receptor member 23 is provided on disc-shaped rotor 8. This urging pressure produces sliding friction between members 22 and 23. The eddy-current-loss and sliding-friction discussed above produce torque loss so that the brushless motor cannot deliver its proper torque. When a brushless motor is designed, this torque loss due to the eddy-current-loss and sliding-friction should be taken into consideration and therefore a greater torque must be built in the motor. For instance, a number of coils or turns of coil is increased, or an outer diameter and thickness of a motor is enlarged; however, these measures generally boost the cost. This is remarkably noticed in the brushless motor for a high speed spinning among others.
If a conventional brushless motor is streamlined by reducing a number of coils, conventional equipment including a coil bonding machine cannot be used anymore, and the depreciation cost thus increases the cost. This blocks lowering the cost of the brushless motor.
Further, a number of turns of coil in each phase is reduced thereby lowering the motor torque.